The present invention relates to a lifting device of the type comprising:
a lower structure,
an upper structure movable with respect to the lower structure between a lowered position and a raised position,
a linkage connecting the upper structure to the lower structure and including at least one articulated arm,
an actuator operatively interposed between the lower structure and the linkage, for controlling the movements of the upper structure between its lowered position and its raised position,
a connecting element between the actuator and the linkage, said connecting element being articulated to said arm of the linkage and being provided with a cam-follower element cooperating with a fixed cam.
A device of the above indicated type is disclosed in JP 2000 238996.
The invention relates in particular to a lifting table with a pantograph linkage of the scissors-type.
As is known, pantograph-type lifting tables enable a movable frame (or platform) to be moved from the lowered position to the raised position while keeping it horizontal, even in case of an off-line mass. Basically, these pantograph-type lifting tables include a fixed base frame, with means for anchoring it to the floor, a movable frame for receiving the articles to be moved and four arms coupled to each other, in a scissors fashion, so as to provide a pantograph-type linkage, which is moved by suitable lifting means.
The lifting table device is shaped so as to occupy the volume of a parallelepiped defined by the two sides of the movable table, whereas its height varies from a minimum value, when the lifting table is closed (platform in the lowered position) up to a maximum value, when the table is opened (platform in its raised position). The pantograph-type lifting tables are particularly useful for automation of large manufacturing processes, also in case of heavy masses to be handled.
The lifting means may be of many types, depending upon the needs and the required forces; for instance, hydraulic cylinders, electric cylinders, or motor and reduction gear units with associated transmissions can be used.
Pantograph-type lifting tables are highly flexible and can be used both as lifting means, or as presses or as pushing devices. Lifting tables, however, have a huge drawback, which is implicit in their own way of operating. Indeed, due to the specific configuration of their linkage, at the beginning of the lifting phase, starting from the closed condition of the pantograph linkage, the vertical movement is hindered by a number of unfavorable leverages, so that the force required for lifting is much greater than the weight to be lifted and is variable throughout the entire movement. In particular, when the pantograph-type table is in its lowered (closed) position, if it has to be lifted by a lifting device operating under the table, during the first lifting step forces that are at least three or four times higher with respect to the actual weight to be moved vertically should be applied. It is evident therefore that there is an interest in developing a lifting device able to exploit all the potential advantages offered by the pantograph lifting tables, while overcoming the above mentioned drawback.
The above mentioned JP 2000 238996 solves the problem only partially, due to the provision of a fixed cam cooperating with a cam-following element carried by the above mentioned connecting element which connects the lifting device to the linkage. However, the arrangement shown in this document is not satisfactory, in particular because the above mentioned connecting element is subjected to a deflecting force during the lifting movement and therefore is not able to transmit the force applied by the actuator with a high efficiency.